The site you used to download Avast from was not the initial reason why your system black screened. That was due to installing Avast while Avira was still installed and running at startup. That was just one example why you should never have two AV's and running in realtime.
Anytime you need to download software, find out who developed the software then go to their site to get the download. If you are ever unsure about the safety of a site for downloads, we're here for you whenever you need us.
It was a pleasure working with you, too!